Figure 4

The surface of \(c^{*}(d_{2},d_{3})\) for the second model. Let \(\alpha =1\), \(\beta =3\), \(S_{0}=1\), \(k=0.2\), and \(\gamma =1.5\) be fixed. Three surfaces from bottom to top are generated by taking \(\tau =4,1,0.2\), respectively. We can see that the minimal wave speed \(c^{*}\) increases with respect to \(d_{2}\) and \(d_{3}\). Moreover, the surface of \(c^{*}\) decreases as τ increases